This invention relates to remote pressure sensing using permanent magnets rotated mechanically by spiral-faced and helical bellows from within sealed vessels, including tires, and exterior sensors responding to magnetic field direction.
Fluor Hanford, Inc. has developed a passive, magnetically-coupled pressure readout system based on a Bourdon-tube mechanism rotating a permanent magnet within a pressure vessel. Bourdon tube mechanisms are fragile, complex, and are not suited for driving appreciable magnet masses nor for overcoming appreciable friction. The latter disadvantages stem from the fact Bourdon mechanisms are relatively weak force wise. In fact, forces generated by the interaction of the earth""s magnetic field with a common permanent magnet can exceed those available from Bourdon tube mechanisms.
The Fluor Hanford device is designed for stationary applications, (spent nuclear fuel canisters) does not contemplate any necessity to compensate for errors due to extraneous magnetic fields. In fact, the device uses a conventional, horizontally oriented, gravity stabilized magnetic compass needle supported by a single-jewel suspension to sense the orientation of the magnet within the pressure vessel.
Angular coupling between rotating elements on shafts via magnetic fields is well known. U.S. Pat. No. 5,382,792 Hurst et al, describes a coupling mechanism wherein permanent magnet pairs are incorporated into coaxial shafts to provide an instantaneous indication of the orientation of a rotating shaft internal to a motor vehicle engine. Such coupling mechanisms employ multiple permanent magnets, oriented pole-face to pole-face. In these types of devices, magnetic coupling between the pole faces of paired permanent magnets aligns the xe2x80x9coutputxe2x80x9d shaft with the xe2x80x9cinputxe2x80x9d shaft. To be effective, such mechanisms require narrow gaps between the pole faces of the respective magnets. These types of devices are hermetically encapsulated for protection from environmental debris and require penetration through the engine wall.
U.S. Pat. No. 3,777,565, Munier et al. describes a sealed water or fluid meter with continuously rotating permanent magnets driven by impellers on input shafts magnetically coupled to magnets on outputs shafts for inducing synchronized rotation. The rotation per unit time of the output shaft indicates the flow rate. Angular displacements (errors) between the xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d shafts are tolerated and even increase torque coupling from the input magnet to the Output magnet.
Numerous devices include mechanisms moving a permanent magnet in response pressure or other force to induce a sensed effect in a material responsive to variation in magnetic field strength. For example, U.S. Pat. No. 4,006,402 Mincuzzi, U.S. Pat. No. 4,843,886 Koppers, et al, and U.S. Pat. No. 4,627,292 Dekrone, each describe a device based on either magnetoresistance and magnetic saturation. U.S. Pat. No. 4,339,955, Iwasaki describes a mechanism that exploits variation in the incremental permeability of a magnetically soft material. Devices based on the sensing the strength of a magnetic field rather than field direction typically require a narrow spacing between the sensor and magnet and are very sensitive to changes in spacing, small misalignments, and extraneous magnetic fields. Accordingly, such devices generally require careful and extensive calibration before measurements are made.
U.S. Pat. No. 4,866,982, Gault describes a tire pressure monitoring system where a stationary Hall-effect sensor measures spacing between a fixed magnet and a second magnet moveable in response to a linear pressure actuator. Changes in spacing between the magnets affect features of the combined magnetic field pattern. Variation in the combined pattern is determined from signal waveforms generated as the spaced magnets, rotating with a wheel, sweep by a stationary sensor. This device requires close coupling between magnet and sensor and penetration into the pressurized interior of the tire and rim.
U.S. Pat. No. 5,814,725, Furuichi et al. describes a mechanism that penetrates through a tire rim wherein a piston-driven screw rotates a permanent magnet. The strength of the magnetic field is detected by a stationary Hall-effect sensor that is mounted transversely to the magnet rotation axis. This type of device typically shares the same problems as the other devices that depend on sensing magnetic field strength.
U.S. Pat. No. 5,047,629, Geist describes a hermetically sealed mechanism for sensing linear displacements of a ferromagnetic armature (e.g., a single turn in a coil spring) according to the attractive force on freely rotating magnet. Disadvantages inherent in this type of device relate to the small distances required between the armature and the magnet, to the small amount of rotational displacement of the magnet produced, and to inadvertent magnetization of the armature.
Other examples of remote pressure reporting mechanisms involve changes in electromagnetic induction or inductive coupling between active elements. For example, U.S. Pat. No. 5,455,508, Takahashi utilizes a form of time-varying (alternating current) electrical excitation. Disadvantages of these types of devices relate to the need to provide a source of operating power within the pressure container and to inadvertent production of eddy currents in nearby conductive materials that will distort the desired field. These types of devices do not sense magnetic field direction.
Still other concepts of remote pressure sensing involve a change the state indicator responding a preset pressure level. For example, U.S. Pat. No. 3,946,175, Sitabkhan describes switching a magnetically susceptible reed in response to pressure actuated displacement of a magnet. U.S. Pat. No. 5,542,293, Tsuda et al. describes a conventional bellows actuated mechanism that uses a fixed and a moveable magnet to switch the orientation of a third magnet. U.S. Pat. No. 5,717,135, Fioretta et al. describes use of magnetic coupling to switch the state of a transducer from producing to not producing a signal. These types of mechanisms are incapable of producing a continuous output responsive to pressure.
Other examples of remote monitoring of vehicle tire pressure involve wireless or telemetric transmission of data. For example, U.S. Pat. No. 5,960,804 McClelland describes a radio transmitter that sends data collected and stored in a memory device within a tire to an external receiver. This active device requires a source of electrical energy (a battery) inside the tire. Alternatively, U.S. Pat. No. 6,053,038 Schramm et al. proposes an external oscillator circuit for generating electromagnetic signals coupling to and energizing a second oscillator within the tire, which changes state responsive to tire pressure and/or other sensed parameters.
Several mechanisms besides Bourdon tubes have been proposed for converting pressure or force into rotary motion. For example, U.S. Pat. No. 4,307,928 Petlock describes a helical bellows for imparting rotational displacement when compressed mechanically in order to make an improved electrical contact. A single, high pitch helical lead is employed because the desired rotational translation is small. U.S. Pat. No. 5,103,670 Wu describes the use of a helical screw to convert linear displacement from a conventional bellows to actuate a directly viewed rotary dial or pointer. U.S. Pat. No. 6,082,170 Lia et al. describes a blood pressure apparatus that uses a diaphragm bellows and a compressible helical ribbon spring to rotate a dial pointer. None of these types of device employs magnetic coupling for remote sensing.
The invented magnetically coupled pressure gauge comprises an internal sender having a spiral-faced or helical bellows coupled for rotating a permanent magnet and an external magnetic sensor responding to magnetic field direction provided by the sender. The invented gauge is particularly suited for remotely measuring pressure within sealed vessels, including vehicle tires, without requiring any penetration through the containing walls of the pressure vessel.
For vehicular applications, a sender secured within a pneumatic tire sweeps past stationary magnetic sensors as the tire rotates. Suitable sensors include induction coils generating signals responsive to the moving magnetic field, and as well, magneto-resistive and/or Hall effect type devices generating signals indicative of magnetic field direction. The orientation of the sender magnet, and hence the pressure, are obtained from the waveforms produced by the induction coils, the magnetoresistive device, or the Hall effect device. In applications where there is little or no relative motion between the pressure sender and the magnetic field direction sensor, magneto-resistive and Hall effect devices are preferable.
The invented magnetically coupled pressure gauge is both robust and economical for remotely measuring, indicating, and monitoring pressure inside a pressure vessel without requiring penetration of the confining walls of the vessel or use an internal power source. In particular, the invented gauge provides passive, non-powered, readout of internally sensed pressure outside a pressure vessel.
Other novel aspects of the invented magnetically coupled pressure gauge relate to mechanisms of compensation for extraneous magnetic fields.
Still other novel aspects of the invented magnetically coupled pressure gauge relate to the pressure responsive mechanisms producing torque for rotating a permanent magnet within a pressure vessel. In particular, the invented gauge provides accurate, remote measurement of pressure over large ranges.
The invented magnetically coupled pressure gauge is particularly suited for passive monitoring of tire pressure within pneumatic tires of motor vehicles. In particular, the invented magnetically coupled pressure gauge can significantly enhance traffic safety in that tire-pressure monitoring and measurement requirements are reduced to operator observation.
Still further advantages of the invented magnetically coupled pressure gauge will become apparent from a consideration of the ensuing description and accompanying drawings.